Low energy particle counter with two-dimensional position sensing

ABSTRACT

A system for detecting radiant energy, such as photons or charged particles, and yielding an output which is indicative of the position in the x, y plane of the input surface of the detection element at which the energy impacts the detection element. The system includes a radiation detector which is subjected to the radiant energy and which yields an output charge in response thereto. The output charge energizes a plurality of charge sensitive elements which give an indication of the position at which the radiant energy impacts the detection element in one direction. A second plurality of charge sensing elements is also energized by the output charge to yield a position indication in the other direction. The individual outputs from the two pluralities of charge sensitive elements can, therefore, be combined to indicate the position along two orthogonal axes at which the detected radiant energy impacted the radiation sensitive element.

United States Patent Somer 51 July 11,1972

[21] App1.No.: 85,593

52 vs. Cl. ..250/83 R, 250/207 [5|] 1m. Cl. ..G01tl/l6 sx Fleld ofSearch ..2s0/41.9o,49.s E, 71.5, 33 0,

250/833 R, 83.3 H, 83.3 HD, 203, 207, 211 K, 220 M, 213 PT; 313/103;356/206, 229

[56] References Cited UNITED STATES PATENTS 3,488,494 l/l970 Tobias..250/49.5 E 3,240,93l 3/l966 Wiley et a1. ..250/4l9 D 3,244,889 4/1966Preston et al ..250/220 M X Primary xaminerWalter Stolwein AssistantExaminer-Davis L. Willis Attorney-Flame, Hartz, Smith and ThompsonABSTRACT A system for detecting radiant energy, such as photons orcharged particles, and yielding an output which is indicative of theposition in the x, y plane of the input surface of the detection elementat which the energy impacts the detection element. The system includes aradiation detector which is subjected to the radiant energy and whichyields an output charge in response thereto. The output charge energizesa plurality of charge sensitive elements which give an indication of theposition at which the radiant energy impacts the detection element inone direction. A second plurality of charge sensing elements is alsoenergized by the output charge to yield a position indication in theother direction. The individual outputs from the two pluralities ofcharge sensitive elements can, therefore, be combined to indicate theposition along two orthogonal axes at which the detected radiant energyimpacted the radiation sensitive element.

9 Claims, 2 Drawing Figures PATENTEDJUL 1 1 1972 3, 6 76 676 sum 10F 2INVENTOR TOIVO A. SOMER BY 6 W ATTORNEY PATENTEDJUL 1 1 I972 3, 676,676

sum 2 OF 2 INVENTOR TOIVO A. SOME TTORNEY LOW ENERGY PARTICLE COUNTERWITH TWO- DIMENSIONAL POSITION SENSING CROSS-REFERENCE TO RELATED CASESThis invention is an improvement of the invention described inapplication Ser. No. 85,592, filed of even date herewith by Toivo A.Somer, entitled, Low Energy Particle Counter With One-DimensionalPosition Sensing, and assigned to the assignee of the instant invention.

BACKGROUND OF THE INVENTION Various types of radiation detection systemsare presently available in the art. Ordinarily, such systems include anelement, such as an electron multiplier, which yields an outputconsisting of an avalanche of charged particles in response to thereception of radiation upon the input surface of the element. Suchdetection systems also include an anode which receives the total chargeexiting from the detection device. The total charge present upon theanode is then detected and measured and is an indication of theradiation which initially impinged upon the input surface to theelectron multiplier Devices for the detection of photons and chargedparticles which are presently available in the art can be divided intotwo categories. The first category includes devices which are useful inthe detection of high energy radiation. In this type of device theradiation sensitive detection element includes a window which isrelatively transparent to the radiation to be detected. Because theradiation is high energy, it passes through the window even though thepassage through the window may attenuate the intensity of the radiation.

The second category of devices includes those which are useful indetecting low energy radiation. This type of device suffers a majordisadvantage because the radiation energy is low, and therefore theattenuation of a window is significant and accordingly frequentlyprevents a useful detection of the energy. Accordingly, the prior artdevices useful in detecting low energy radiation ordinarily do notinclude a window. Instead the low energy radiation impinges directlyupon the energy detection device, thereby eliminating the attenuationwhich would ordinarily be suffered because of the presence of thewindow. The instant invention falls into either of these categories ofradiation detection systems.

SUMMARY OF THE INVENTION The inventive system is capable of yielding anoutput which is indicative of the position of an impact of radiantenergy upon the radiation detection element of the system. The positioninformation is given in two dimensions with respect to surface of aplane. One embodiment of the inventive system includes an array ofconductive parallel line anodes, which are spaced along one dimension ofthe detection device. Each of the parallel anodes is connected to anoutput indication device so that an output from one of the individualparallel anodes is indicative of the impact of a particle with oneconductive anode. Because the location of the anode is known, thelocation of the impact in one direction along the surface of thedetection element is also known.

A second array of parallel line anodes is spaced at a a suitabledistance from the first array and is arranged so that the individualanodes within the two arrays are perpendicular to one another. The twoarrays of parallel line anodes are at different potentials so that thecharged particles impinging upon the first array will pass through thisarray and impinge upon the individual conductors of the second array.

Suitable output indication means are connected to the individualconductors of the second array so that an output from one of the readoutmeans is indicative of an impact with a particular conductor within thesecond array and consequently the positioning of the impact of radiantenergy along the other dimension of the radiation sensitive element isknown.

The inventive system is, therefore, capable of yielding an output whichis indicative of the positioning in two dimensions of the impact ofradiant energy upon the multiplier input surface. Also, by counting theoutputs received from all the parallel conductors within one of thearrays, the total number of particles which impinge upon the detector ina particular period of time can be determined.

Another embodiment utilizes a planar conductive surface which extendsalong the entire output surface of the detection element. A plurality ofparallel linear resistive elements are physically displaced from theconductive surface so that charged secondary emission particlesemanating from the output surface of the detector pass through theconductive layer and impinge upon the resistive elements.

The resistive elements are grounded at one end and virtually grounded atthe other end. The virtual grounding is achieved by connecting each ofthe resistive elements to a low input impedance amplifier. Because ofthe grounding of both ends of the resistive elements, charges impactingwith the elements split in accordance with the resistive ratio betweenthe point of impact and the total resistance of the elements. Theproportional charge serves as an input to a two input divider.

An output which is independent of the statistically variable totalcharge is obtained by inputting the total charge into the divider. Theproportional charge is then divided by the total charge so that thetotal charge factor is eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION The preferredembodiment of the invention shown in FIG. 1 includes a photomultiplier10, of a type known in the art. Radiant energy, indicated by the arrow11, and which can be photons or charged particles, impinges upon theinput surface 12 of the photomultiplier 10. In response to theimpingement of the radiant energy 11, the electron multiplier 10produces an avalanche of electrons which are ejected from the outputsurface 13 of the multiplier, because of the potential difference of VAn exemplary photomultiplier which can be used is a spiraltron matrix,which is described in pages 376-380 of the IEEE, Transactions on NuclearScience," Volume NS-16, No. 1, February, 1969, in an article entitled,Spiraltron Matrixes as windowless Photo Detectors for Soft X-Ray andExtreme U. V.," by T. A. Somer and P. W. Graves.

An array of conductive parallel anodes 14 is evaporated or otherwiseapplied to the photomultiplier output surface 13 to permit outputdetection along the lengths of the parallel line anodes. The parallelline anodes 14 are placed at a potential which is more positive than thepotential of the input surface 12 of matrix 10.

A suitable output detection means 15 of a type well known in the art,such as a pulse amplifier-counter, is connected to each of theindividual parallel line conductors 14 so that the charge present oneach conductor is individually detected. Accordingly, by properlyidentifying the parallel line conductors 14, for example alphabeticallyas shown in FIG. 1, the position along an axis at which the chargesleave the output surface 13 is indicated by the readings on theindividual readout means 15. The total number of charges which impingeupon the several conductors forming an array in a selected time periodcan be determined simply by counting the number of indications presentedby the detectors 15 in the selected time period.

A second array of parallel line conductors 16 is physically spaced fromthe first array of parallel conductors 14. The individual conductors ofthe second array are numbered I to IX.

The array of parallel line conductors 16 can be physically supported inthe spaced relationship by an insulating or dielectric member which isof a type within the purview of one skilled in the art.

A voltage source V, which is used to maintain the second array ofparallel conductors 16 at a more positive potential with respect to thefirst plurality of parallel conductors 14, is arranged across the twoarrays of parallel conductors. Accordingly, the electrons emanating fromthe output surface 13 of the photomultiplier will pass through theconductors 14 and impact with the conductors 16. Consequently, thecharges impacting the various conductors I-IX will yield outputindications on the output readout means 17. These outputs are indicativeof impact positions along the y axis of the input surface 12 of matrix10. Accordingly; by using the two arrays of parallel conductors and byperpendicularly arranging the individual conductors in the two arrays sothat a grid-like elementis formed, it is possible to determine the exactposition along the x and y axes of the output surface 13 from which theelectrons emanated. This position indicates the position on the inputsurface 12 at which the initial energy impacted with the photomultiplier16. As an example, if a reading is simultaneously present on the x axis,conductor G and the y axis conductor V, it is immediately known that anelectron emanated from the output surface 13 in the position identifiedas G-V. Radiation must, therefore, have impinged upon the matrix 10 at acorresponding point on input surface 12. It is, therefore, possible tovery precisely locate the position in two dimensions at which theradiant energy impinged upon the input surface 12 of the photomultiplier10. Furthermore, by counting the number of readouts obtained from allthe conductors of one of the arrays of parallel conductors obtained in aselected time period, it is possible to determine the total number ofparticles which impacted with the input surface 12 of thephotomultiplier 10 during the selected time period. The inventive systemis therefore a significant step forward in the art, in that it yields atwo-dimensional position indication, as well as a total particleimpingement indication.

The embodiment of FIG. 2 is similar to that of FIG. 1 in that itutilizes the same photomultiplier 10 and a similar input surface 12.However, the output surface 21 is covered with a continuous planarsurface 21 so that the secondary emission charges emanating from theoutput surface 13 are not indicative of the position from which thecharged particles emanate. Accordingly, the secondary emission chargesemanating from the output surface 21 cause a voltage on the lead 22which is electrically connected to a corresponding input on the analogdivider 27. The lead 22 is maintained at a potential which is morepositive than the input surface 12 of the photomultiplier l0.

Physically displaced from the planar output 21 is a plurality ofparallel linear resistance elements 23. One end 24 of each of theparallel linear resistive elements 23 is grounded as indicated at 25.The other end of each of the linear resistive elements 23 isindividually connected to an amplifier 26 having very low, oressentially zero, input impedance. Each of the low impedance amplifiers26 is connected to one of the inputs of dual input analog dividers 27.Accordingly, each of the analog dividers 27 receives an input which isindicative of the charge impinging upon one of the parallel linearresistive elements 23. That is, the resistive element 23A serves as aninput to the analog divider 27A while the resistive element 23B servesas an input to the analog divider 278. Accordingly, the position alongthe x axis of the output surface 21 of the photomultiplier 10 at which acharged particle emanates is evidenced by an indication on one of theoutput leads of an analog divider 27. As an example, an output on outputlead 28A of analog divider 27A indicates that. an electron impacted theresistive element 23A. This is an x axis position indication of animpact on the input surface 12 of the photomultiplier 10.

The y axis position indication is obtained from the use of chargedivision techniques which are possible because of the utilization oflinear resistive elements for the parallel conductors 23. Each of theanalog dividers 27 has a second input which is connected to the outputlead 22 which receives a voltage because of the secondary emissioncharges emanating from the output surface 21 of the photomultiplier.Each of the linear resistive elements 23 is grounded at one end, asindicated by reference number 25. The other end of each of the linearresistance elements 23 is connected to an amplifier 26. The amplifiers26 have virtually zero input impedance and, therefore, both ends of theconductors 23 can be considered as grounded. The total charge impactingeach of the individual linear resistive elements 23 is therefore dividedin accordance with the relationship:

where:

Y the distance from the grounded end 24 to the position at which theelectron impacts the linear resistive element 23; I

L= the total length of the conductor; and

Q= the total charge striking that conductor.

Because the amplifiers 26 have essentially ,zero input impedance, thecharges striking each conductor divide in accordance with the Y/L ratio.Consequently, by utilizing the analog dividers 27 to divide the totalcharge 0 received by anode 21 into the (Y/L)Q ratio, the dependence ofthe output upon the charge Q is eliminated. The output voltage of eachdivider is indicative of the Y position along the various linearresistive elements 23, at which energy impinged upon the matrix inputsurface 12.

The output of each of the analog dividers 27 is therefore indicative ofthe two-dimensional position of the impact of radiant energy upon theinput surface 12 of the photomultiplier 10. For example, an output onoutput lead 28A of divider 27A immediately indicates that energyimpacted the input surface 12 of the photomultiplier 10 somewhere alongthe length of the linear resistive element 23A. This yieldsthe xcoordinate information relative to the impact. The y coordinateinformation is obtained from the amplitude of the pulse present on theoutput lead 28A.

The embodiments of the inventive system illustrated in FIGS. 1 and 2 aretherefore seen to be advantageous over the art systems in that theyyield two-dimensional position information relative to the position atwhich radiant energy impacts the input surface of a spiraltron matrix ora photomultiplier.

The embodiment of FIG. 2 is advantageous over that of FIG. 1 because theneed for the x axis readout means 15 is eliminated. However, FIG. 2requires the use of dividers 27 and amplifiers 26, which may in someinstances be undesirable. The total teaching of the two embodiments isvery flexible, allowing a selection of methods which is mostadvantageousin view of the particular application and availability of elements.

What is claimed is:

1. A two-dimensional position indicating system for detecting radiantenergy comprising:

an energy detection element having an input surface and an outputsurface, a plurality of charged particles emanating from said outputsurface in response to the impact of a single particle of radiant energywith said input surface;

a first plurality of spaced conductors in the proximity of said outputsurface and extending completely along one dimension of said outputsurface, said conductors being subjected to said charged particlesemanating from said output surface;

a second plurality of spaced conductors extending completely alonganother dimension of saidoutput surface and perpendicular to said firstplurality of conductors, and physically displaced from and in theproximity of said first plurality of conductors, said second pluralityof conductors being subjected to said charged particles as they passthrough said first conductors; and

readout means responsive to the charges impinging upon said first andsecond conductors to yield an output indicative of the two-dimensionallocation at which said radiation impacted said input surface.

2. The system of claim 1 wherein said readout means includes a firstplurality of readout units equal to said first plurality of conductorsand individually responsive to the outputs of said first conductors sothat individual indications of said first readout units are indicativeof the position of impacts of energy with said input surface along saidone dimension; and

a second plurality of readout units equal to said second plurality ofconductors and individually responsive to the outputs of said secondconductors so that individual indications of said second readout unitsare indicative of the position of impacts of energy with said inputsurface along said another dimension.

3. The system of claim 1 wherein said first plurality of conductors areindividually identified along a y axis of said output surface and saidsecond plurality of conductors are individually identified along an xaxis of said output surface so that each charged particle emanating fromsaid output surface impacts one conductor in each of said first andsecond pluralities and yields outputs respectively having an x-yaddress.

4. The system of claim 1 wherein said energy detection element is anelectron multiplier and said first plurality of spaced conductors isevaporated onto said output surface in the configuration of uniformablyspaced parallel conductive lines arranged perpendicular to said secondplurality of conductors.

5. A two-dimensional position indicating system for detecting radiantenergy comprising:

an energy detection element having an input surface and an outputsurface, a plurality of charged particles emanating from said outputsurface in response to the impact of a single particle of radiant energyupon said input surface;

a plurality of linear resistive elements having a length extending alongone dimension of said output surface and arranged along the, otherdimension of said output surface, said resistive elements beingphysically displaced from and in the proximity of said detectionsurface, said resistive elements also being responsive to said chargedparticles as they pass through said detection surface, said resistiveelements serving to split the charges received from said detectionsurface in accordance with a ratio determined by the linear position atwhich said charges impact said resistive elements; and

a plurality of readout means individually responsive to said linearresistive elements and responsive to said detection surface to yield anindication of the two-dimensional locations at which said chargedparticles emanate from said output surface.

6. The system of claim 5 wherein said readout means in- 5 cludes aplurality of low input impedance amplifiers individually responsive tosaid resistive elements;

a plurality of dividers individually responsive to said amplifiers sothat an output from one of said dividers is indicative of the positionof energy impact on said input surface along one dimension of said inputsurface and the amplitude of said divider output is indicative of theposition of energy impact with said input surface along a seconddimension of said input surface.

7. The system of claim 6 wherein one end of each of said resistiveelements is directly grounded and the other end of each of saidresistive elements is relatively grounded through one of saidamplifiers; and said dividers are two input analog dividers.

8. The system of claim 6 wherein one input of each of said dividers isresponsive to said conductive surface and the other input of saiddividers is individually responsive to said resistive elements.

9. The system of claim 8 wherein said ratio is Y/ L where:

Y the distance between the point of input of said particle and saidgrounded end;

L= the total length of said resistive element.

1. A two-dimensional position indicating system for detecting radiantenergy comprising: an energy detection element having an input surfaceand an output surface, a plurality of charged particles emanating fromsaid output surface in response to the impact of a single particle ofradiant energy with said input surface; a first plurality of spacedconductors in the proximity of said output surface and extendingcompletely along one dimension of said output surface, said conductorsbeing subjected to said charged particles emanating from said outputsurface; a second plurality of spaced conductors extending completelyalong another dimension of said output surface and perpendicular to saidfirst plurality of conductors, and physically displaced from and in theproximity of said first plurality of conductors, said second pluralityof conductors being subjected to said charged particles as they passthrough said first conductors; and readout means responsive to thecharges impinging upon said first and second conductors to yield anoutput indicative of the two-dimensional location at which saidradiation impacted said input surface.
 2. The system of claim 1 whereinsaid readout means includes a first plurality of readout units equal tosaid first plurality of conductors and individually responsive to theoutputs of said first conductors so that individual indications of saidfirst readout units are indicative of the position of impacts of energywith said input surface along said one dimension; and a second pluralityof readout units equal to said second plurality of conductors andindividually responsive to the outputs of said second conductors so thatindividual indications of said second readout units are indicative ofthe position of impacts of energy with said input surface along saidanother dimension.
 3. The system of claim 1 wherein said first pluralityof conductors are individually identified along a y axis of said outputsurface and said second plurality of conductors are individuallyidentified along an x axis of said output surface so that each chargedparticle emanating from said output surface impacts one conductor ineach of said first and second pluralities and yields outputsrespectively having an x- y address.
 4. The system of claim 1 whereinsaid energy detection element is an electron multiplier and said firstplurality of spaced conductors is evaporated onto said output surface inthe configuration of uniformably spaced parallel conductive linesarranged perpendicular to said second plurality of conductors.
 5. Atwo-dimensional position indicating system for detecting radiant energycomprising: an energy detection element having an input surface and anoutput surface, a plurality of charged particles emanating from saidoutput surface in response to the impact of a single particle of radiantenergy upon said input surface; a plurality of linear resistive elementshaving a length extending along one dimension of said output surface andarranged along the other dimension of said output surface, saidresistive elements being physically displaced from and in the proximItyof said detection surface, said resistive elements also being responsiveto said charged particles as they pass through said detection surface,said resistive elements serving to split the charges received from saiddetection surface in accordance with a ratio determined by the linearposition at which said charges impact said resistive elements; and aplurality of readout means individually responsive to said linearresistive elements and responsive to said detection surface to yield anindication of the two-dimensional locations at which said chargedparticles emanate from said output surface.
 6. The system of claim 5wherein said readout means includes a plurality of low input impedanceamplifiers individually responsive to said resistive elements; aplurality of dividers individually responsive to said amplifiers so thatan output from one of said dividers is indicative of the position ofenergy impact on said input surface along one dimension of said inputsurface and the amplitude of said divider output is indicative of theposition of energy impact with said input surface along a seconddimension of said input surface.
 7. The system of claim 6 wherein oneend of each of said resistive elements is directly grounded and theother end of each of said resistive elements is relatively groundedthrough one of said amplifiers; and said dividers are two input analogdividers.
 8. The system of claim 6 wherein one input of each of saiddividers is responsive to said conductive surface and the other input ofsaid dividers is individually responsive to said resistive elements. 9.The system of claim 8 wherein said ratio is Y/L where: Y the distancebetween the point of input of said particle and said grounded end; L thetotal length of said resistive element.